Different types of flow measuring devices based on the principle of generating oscillating jet are known. These type flow meters generate frequency signals proportional to the flow of fluids. These recirculation type flow meters currently use sensors and sensing methods which are not very accurate and are affected by noise.
There are several methods of flow conditioning and devices available such as tube bundles, perforated plates or wire mesh screens acting as flow conditioners. Conditioners of simple straight pipe length may be sufficient. However, where space is a constraint and flow profile is disturbed, then there exists a need to have a compact flow conditioner suitable for rectangular nozzle flow forming a jet.
U.S. Pat. Nos. 3,902,367 and 4,838,091 disclose flow meters wherein the flow rate is determined by measuring the amplitude and frequency of the oscillations. Hence, these flow meters are not very accurate.
U.S. Pat. No. 7,383,740 discloses a spirometer. The spirometer uses pressure and velocity sensors to detect the flow rater of the fluid. This results in inaccuracies and errors as pressure and velocity do not indicate the exact flow rate.
PCT application WO2006114592 describes a flow meter provided with means to apply magnetic field and electrodes. This makes the flow meter complicated and also prone to errors.
U.S. Pat. Nos. 5,983,943 and 6,321,790 disclose flow meters using pressure sensors or differential pressure sensors for determining the fluid flow rate.
U.S. Pat. No. 6,606,915 discloses a flow meter, using temperature sensitive elements as sensors. The change in resistance of a heated sensor is used to detect frequency of oscillations according to the meter disclosed in this document.
U.S. Pat. No. 7,094,208 discloses a flow meter which determines the flow rate using thermistors or other such temperature sensitive elements as sensors.
PCT application 2008110766 describes a bi-directional flow meter. The measurement means typically used are inductive sensors, ultrasonic sensors or pressure sensors. The frequency of the output signal is proportional to the frequency of oscillations, which in turn is used to determine the flow rate of the fluid.
Following are the limitations of flow meters of the prior art:
When magnets are used for creating magnetic field for sensing oscillation frequency of fluid, the fluid must have certain electrical conductivity. Hence, the flow of oils and gases cannot be measured. If magnetic particles are present in the flowing fluid, the meter will immediately get clogged since the magnets used are very strong and the burr or magnetic material once attracted towards the magnet are impossible to be removed. In addition to that, the magnets are costly and the assembly of these flow meters is very difficult. Also, they have limited operating temperature range.
When temperature sensitive type sensors are used, these sensors are heated above the operating temperature of the fluid and the oscillatory flow is sensed by cooling and heating periodically. These sensors are very delicate and external power is required to heat up the sensor. To compensate for frequency response of the sensor to varying operating temperature and other fluid properties of the flowing fluid, it is necessary to have complicated electronic circuitry for proper and accurate sensing of fluid fluctuations. Thus these meters have a limited operating pressure and temperature range.
Flow meters based on differential pressure measurement are also used currently. Fluctuations in the differential pressure measured across the feedback path or main stream can be used to measure frequency of oscillatory flow. However, the sensing ports, tubes and the volume across the diaphragm of the sensor add to the hydraulic capacitance which actually increases the volume effectively and add to the delay in feedback sensing. Also, the actual signal is attenuated to a large extent. This type of sensor also needs external power for its operation. If the tubes and ports are kept very small to avoid these problems, then a very small particle in the flow can block the sensor ports. In the medical applications, where the sterility of the instrument is a very essential factor, this tubing and passages can pose serious threat since it is not possible for them to get cleaned thoroughly. Further, these flow meters also have limited operating pressure and temperature ranges.
Inductive sensors have also been used in flow meters. The inductive flow meters are very sensitive to noise and hence not very accurate. The arrangement of coils and movable core is similar to a probe commonly known as the L.V.D.T. probe. A set of coils is excited by an ac voltage signal and a movement of the core inside the coils generates an electrical signal in another set of coils. To sense the fluid's oscillatory frequency, the probe with sealing arrangement, having a diaphragm or oil filled bellows, is located in the flow meter. The diaphragm moves in response to the oscillating fluid pressure, wherein the core is connected to the diaphragm on the other side and thus the signal is sensed. These types of sensors perform poorly at low frequency and at high frequency due to mechanical inertia.
Therefore there is a need for a flow meter which is more accurate and insensitive to noise.